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The Q-Bar on the frame rate/connection status meter area now has two side-by-side meters.
The original value is in the left-hand bar, and shows the amount of packet loss from the race server to the simulation.
The new right-hand bar (which grows from right to left) shows the amount of packet loss from the simulation to the race server.
Note that the data for this right-hand bar is sent from the server to the simulation, so if there is a sudden loss of all packets sent by the server to the simulation, the value in this bar will not update. As before, each bar becomes full at 50% packet loss, though each is now about half the width of the original Q meter.
See FPS / Network display and FFB (see LQSPF in Rookies)

We have enhanced our Netcode with an option for greater bandwidth, allowing more information to pass between you and our servers
Two new Connection Type selections are now available on the main Account->My Account page, under Preferences.
These selections allow higher network bandwidth usage between the simulation and the race servers.
Mostly, this allows the server to send more detailed data for each opponent car and/or more opponent cars to the simulation.
If your computer, graphics card, and Internet connection are all fast enough, and you select one of these new options, you’ll likely also want to adjust the in-sim Options->Graphics “Max Cars” setting to take advantage of the extra allowed bandwidth usage.

- Fixed a couple of issues that could cause small amounts of packet loss to briefly generate higher apparent latency and lower quality.

- Our US ISP has resolved the issue that was preventing proper IPv6 routing switch-over to redundant equipment in the event of an equipment failure. We have re-enabled IPv6 for Simulation to Race Server connections on the US server farm

- Fixed an issue where sometimes the server would not use the space available to transmit opponent cars to you, leading to cars disappearing, especially when you were on or near pit road.

- Fixed an issue that was incorrectly interpreting the values implied by your "My Account/Preferences/Connection Type" selection. This incorrect interpretation made the system think you had less available bandwidth than your selection. This could have led to less data being sent to you, resulting in the display of a fewer number of cars and possibly bad or missing voice chat.

- Prevent the Sim from dynamically probing your network connection to see how large a packet it can send.

- Changed the default maximum network packet size to ensure that Connection Type selections on the Account->My Account->Preferences page on the web site up to “512K or faster” require only one packet. The two higher selections (768K and 1Mbit) will require more than one packet.

From the superceded patch 1 release notes - check accuracy:
- The default initial maximum size of a network datagram the Sim generates has been adjusted. At this new default size, maximal packets using the "512k or faster" bandwidth selection will require only one datagram to be sent in full. In the "core.ini" file, the "[Communications] default_datagram_max_size" setting has been renamed "default_dgram_max_size" to ensure this new default is used.

- Dynamic probing of the maximum size datagram your connection will support has been disabled. With this probing disabled, the number of datagrams needed to send a maximal "1Mbit/sec or faster" packet increases from 2 to 3. In the "core.ini" file, the "[Communications] enable_path_mtu_discovery" setting has been renamed to "enable_pmtu_discovery" to ensure this new default is used.

Related post by iRacing staff, Randy C:
The release note about this could have been wordier.

We can send information about the movement/status of opponent cars to you with different levels of detail. The more-detailed information lets us replicate the movement of the opponent cars more accurately (among other things), but it takes more bandwidth. If, given your bandwidth selection, not all the cars to be sent will fit using the more-detailed information, then some of them will be sent with less-detailed information. The cars sent to you with less-detailed information can cause worse "net code" (though to a lesser degree than would higher latency or higher packet loss).

There are two "knobs' you can use to tune things. One is the web site's Account->My Account->Preferences->Connection Type setting, which lets you control the maximum amount of network bandwidth the sim will use. And the other is the in-sim Options->Graphics->Max Cars setting, which lets you limit how many opponent cars are sent to you.

Since there should be almost no CPU or Graphics hit to receive bigger blocks of data, you can turn the "connection type" knob based on your Internet connection. Turning this knob "up" means there's enough room to send cars to you. We adjust the ratio of "more-detailed" cars to "less-detailed" cars to ensure as many cars will fit as there is room for.

However, there can be a significant CPU and frame rate hit to process more opponent cars (doing their physics, generating their sounds, handling their collisions, drawing them and the smoke/dust/debris they can generate, etc). So if you find that after turning up the connection type knob your frame rate suffers, or your CPU usage goes too high, you can turn down the "max cars" knob to help reduce those issues. Since that makes it so there are fewer cars to be sent to you, while the space available to send them remains the same, the ratio of "more-detailed" to "less-detailed" cars can go up. Meaning you still benefit from the more accurate opponent car positioning (better "net code", without taxing your CPU or graphics card.

We made it so that if you choose the highest bandwidth selection ("1Mbit/sec" ), all cars can be sent using the more-detailed information.
With the "768K" selection, you start getting the less-detailed information at around 48 cars that are transmitted to you.
With "512k", it's at around 24 cars.
With "256k", it's at around 18 cars.
With "128K", it's at around 12 cars.
This message was edited 2 time(s). Last update was at 3/19/2016 9:24 a.m.

Added...
If it can't use the more-detailed representation for all the cars, it will use the more-detailed representation for the cars close to you, and the less-detailed representation for the cars that are farther away from you.

Another post by Randy C:
The numeric values in the selections say what your minimum rated download speed should be. Clearly, the sim is going to "upload" data, too, but it's far less data. The three bandwidth selections that existed prior to this season all have the same max upload rate - about 100kbit/sec. The two new bandwidth selections could potentially use more upload bandwidth than that, though there aren't many situations where it will actually use much more. With the 768kbit/sec selection, it could potentially upload around 130kbit/sec, and with the 1Mbit/sec selection it could potentially upload around 190kbit/sec.

Posted by iRacing staff, Randy C (Feb 2016):
The simulation uses a tiny amount of bandwidth.
From lowest to highest selection on the Account/My Account/Preferences/Connection type page on the web site:
"DSL, Cable, Fiber, 128K or faster" - 110kbits/sec download, 85kbits/sec upload
"DSL, Cable, Fiber, 256K or faster" - 150kbits/sec download, 100kbits/sec upload
"DSL, Cable, Fiber, 512K or faster" - 180kbits/sec download, 100kbits/sec upload
The download values are the maximum bandwidth amounts used in fairly full sessions. The upload numbers are maximums that should should rarely be hit - it should generally be no more than about half that. EasyAntiCheat will use a little bit of bandwidth, too, but, in theory, it shouldn't be more than about 5kbits/sec once you get connected.

If you are using Trading Paints, that will probably download the paint jobs of the other drivers in your session. Its bandwidth usage is beyond our control.

You might try bringing up the Windows Task Manager just before entering the simulation, and selecting its "Networking" tab. On Windows 7, select View->Network Adapter History, and enable all three (bytes sent, bytes received, bytes total). Divide the numbers above by 8 to convert from "bits" to "bytes", and then multiply by 1000. The Task Manager numbers should not exceed those. If they do, then something else on your computer is using bandwidth.

We use IPv4, unless you have native IPv6 connectivity available on your computer and you have explicitly told the sim to use it by enabling it on the Settings page on the web site. Nobody that has posted in this thread before my post has ever used IPv6 to connect the sim to our race servers. (Only about 1.2% of connections to our race servers have been via IPv6 this year.)

A few posts later, reference to replay spooling and OneDrive.
Tapani suggested: "If you have OneDrive running, right click OneDrive, Settings -> Auto-save -> Documents: This PC only. Should fix this, if Randy is correct."

New Netcode
We have implemented some new netcode that sends more data to and from each car.
This allows us to utilize more information for each vehicle, such as the pit limiter status for all cars that use pit limiters, and the DRS wing state and ERS warning light for the McLaren MP4-30.

To help account for the additional data that can be sent for each car, the amount of network bandwidth that can be used for sending data from the server to the simulation has been increased by about 12% to 108kbits/sec for the “128K or faster” connection type selection, and by about 9% to 142kbits/sec for the “256K or faster” connection type selection.

See the Preferences section on the Account->My Account page to select your connection type.

Update July 2015 post by Randy C.
Condensed notes:
(see 2016s1 update note above re increased data)
Usage depends on setting in My Account/Preferences/Connection type, number of cars, and voice chat.
Data downloaded from iRacing server to us:
"512K" connection selected: could amount to ~145Kbits/sec with nobody talking on voice chat, or ~63 megabytes/hour.
"256K" connection type selected: multiply that by about 2/3, so about 42 megabytes/hour. (Ed: why 2/3?).
"128K" connection type selected: it's about 42% of the 512K number, or about 26 megabytes/hour.
Voice chat: If people are talking a lot, you could add maybe 500 Kbytes to 1 megabyte per hour.
Uploads (data from us to iRacing): possibly up to 20 megabytes/hour.
Update October 2015 post by Randy C.
For dynamic tracks info add about 23kbits/sec from client to server, and about 12kbits/sec from server to client.
(It might take less at times, but allow for that).
Connection typepost by Randy C (11/2015).

The setting basically changes the maximum size of packets that can be
sent from you to the server, and from the server to you.
Larger packets
means more bandwidth could be used, and more space is available for
sending additional cars, though it also means that some other "control" type of information (like when a new person joins the server) can get through more quickly.
Aside from the now-retired "dialup" selection, there is no change in voice chat or dynamic track information with any of the selections.
The "requires fast computer"
language is there because when more data get sent to you, it takes more
CPU to process it. In particular, it takes more CPU to move around
additional cars, generate their sounds, handle collisions they're
involved in, etc. The "Max Cars:" setting on the Options/Graphics page can be used to reduce the number of cars that are sent to you.